Protective device



Nov. 4, 1947. E, F. w. BECK ET AL PROTECTIVE DEVI CE Filed June 27, 1944WITNESS-ES:

& n M 306 ed m rm W S mw mm J ATTORN EY Patented Nov. 4, 1947 UNITEDSTATES PATENT OFFICE PROTECTIVE DEVICE tion of Pennsylvania ApplicationJune 27, 1944, Serial No. 542,404

4 Claims.

The present invention relates to protective gap devices, and moreparticularly to a protective gap for heavy-current discharges in whichthe electrodes of the gap weld, or fuse together.

The protective gap of the present invention is especially adapted forapplications where protection is required against voltage surges such asmay be caused by switching operations or circuit interruption, and Where very heavy currents, such as currents of the order of severalthousand amperes, may flow through the gap for appreciable periods oftime. In such applications, and especially on direct-current circuits,it is not practical to use a self-restoring device which is capable ofinterrupting the heavy current, and it is desirable to utilize a devicein which the electrodes of the gap weld, or fuse together, to provide acontinuous metallic path which is capable of carrying the currentwithout burning or excessive heating. Such a device, of course, must bereplaced after it has operated, but this is not objectionable sinceoperation of the gap occurs relatively infrequently, and as a result ofknoWn conditions such as switching operations.

The principal object of the invention is to provide a protectivedischarge gap which breaks down and becomes conducting upon theoccurrence of a predetermined overvoltage, and in which the electrodesweld together to form a continuous metallic path which is capable ofcarrying heavy currents.

More Sp c fically, the object of the invention is to provide aprotective discharge gap for heavycurrent discharges in which theelectrodes weld together when a discharge occurs, and which is capableof discharging very heavy currents without overheating, and withoutcontinued arcing, emission of flame, or other noticeable disturbance.

Other objects and advantages of the invention will be apparent from thefollowing detailed description, taken in connection with theaccompanying drawing, in which:

Figure 1 is a transverse sectional View of a gap device embodying theinvention, approximately on the line II of Fig. 2, and

Fig. 2 is an end elevation of the gap shown in Fig. 1.

In the preferred embodiment of the invention shown in the drawing, theprotective discharge gap device comprises two closely spaced metalelectrodes'l and 2, which are preferably made of copper. Each electrodehas an annular groove 3 near its periphery to confine the discharge tothe centralplane sparking surface 4, and has an annular recess aroundits outer periphery to increase the creepage distance between theelectrodes and prevent discharges across the outside of the gap. Thesparking surfaces 4 are of relatively large area so as to be capable ofcarrying heavy discharge currents, and the electrodes are preferablymade relatively thick and massive so as to have high thermal capacity.The large sparking surfaces also insure consistency of the break-downvoltage of the gap. 'The electrode I has a recess 5 formed in the centerof its sparking surface and surrounding a hole "I for the reception of abolt 8. The electrode 2 has a central opening 9, which may be of largersize, and which has its edges beveled, as indicated at Ill, to coincidewith the recess 6 in the electrode I, so that the recess 6 and opening 9together form an open chamber in the center of the sparking area of thegap. The electrodes are separated and spaced apart by an annular washerII of mica, or other suitable insulating material, which insulates theelectrodes from each other under normal conditions, and which spacestheir sparking surfaces 4 the desired distance apart. The washer IIcompletely encloses the sparking surfaces so as to prevent the entranceof dirt or other contaminating substances.

The gap device is mounted in position by means of mounting brackets I2and I3, which are also used to effect electrical connection to theelectrodes, and the brackets I2 and I3 are preferably secured to theelectrodes I and 2, respectively, by means of screws I4. An insulatingbushing I 5 is provided, having a portion I6 which is adapted to extendthrough an opening in the bracket I3 and into the opening 9 in theelectrode 2, and an outer portion of larger diameter which extends overthe outer surface of the electrode and the bracket I3. The bushing I5has a central hole IT for the reception of the bolt 8. The gap device isheld together by means of the bolt 8 which, as shown, passes through thebracket I2 and the hole I in the electrode I and through the hole I! inthe bushing I5, which insulates the bolt from the electrode 2. A washerI8 is placed in a recess in the bushing I5, and a nut I 9 on the bolt 8bears against the washer I8, a lock washer 29 being preferably alsoprovided to prevent movement of the nut I9 after the gap has beenassembled, which might change the spacing between the electrodes. Therecessed outer portion of the bushing I5 provides a long leakage pathbetween the bolt 8 and the bracket I3, while the inner portion I S ofthe bushing similarly provides a long leakage path between the bolt 8and the electrode 2, the latter leakage path being protected againstdirt, or other conducting or semiconducting particles which might reducethe resistance to leakage currents. The bushing 15, in conjunction withthe bolt 8, also serves to align the electrodes and thus facilitatesassembly of the gap.

Since the electrodes are intended to weld together when the gapoperates, they are closely spaced, the exact spacing depending, ofcourse, upon the desired breakdown voltage. Thus, in a typicalembodiment of the invention, the thickness of the mica washer H may bemade such that the sparking surfaces 4 of the electrodes I and 2 arespaced apart a distance of the order of to 25 mils, so that the gapbreaks down on a voltage of the order of 2000 to 3000 volts. When avoltage in excess of the predetermined breakdown voltage appears acrossthe gap, it will commence discharging, and with a heavy dischargecurrent, the electrodes l and 2 will weld together almost immediately,so that the heavy current is discharged to ground through a continuousmetallic path and the device does not become overheated, and nocontinued arcing or emission of flame or other undesirable phenomenaoccur.

The provision of the central space or chamber between the electrodes,formed by the opening 9 and recess 6, is an important feature of theinvention, since it has been found that unless this space is provided,the electrodes will not weld together in the desired manner. Our presenttheory of the necessity of this space for satisfactory operation of thegap is that when a heavy-current discharge occurs, the heated gas andmetal vapor in the restricted space between the electrodes tend toincrease in pressure very rapidly because of the high temperature causedby the heavy current, and because of vaporization of the metal. If anexpansion space is provided in the center of the sparking area of thegap, these heated gases and metal vapors can expand into this space andrelieve the pressure in the sparking area, and welding of the electrodesoccurs rapidly. If no such space is provided, however, the pressurebetween the electrodes can not be relieved and rapidly builds up to sucha high pressure that welding of the electrodes is prevented, and the arctravels around between the electrodes, causing excessive heating andemission of flame from the gap. High pressure is also likely to blow theare out to the outside of the mica spacer. These effects, of course, arehighly objectionable, and a gap which does not weld, and in whichcontinuous arcing and emission of flame or other visible disturbancesoccur is not satisfactory.

Whatever may be the theoretical reason for the necessity of theexpansion space between the electrodes, however, our tests have shownconclusively that satisfactory operation can not be obtained unless sucha space is provided. If it is provided, welding of the electrodes occursalmost immediately upon the occurrence of a discharge, and the operationis entirely satisfactory. Thus, in one particular embodiment of theinvention, a gap of the construction shown in the drawing was designedto withstand 700 volts direct current continuously, and to commencedischarging at a voltage of about 2400 volts direct current. Theelectrodes of this gap welded together substantially immediately uponthe occurrence of a discharge, and the gap was capable of carrying 7500amperes for as long as 30 seconds without excessive heating and withoutnoticeable arcing or other objectionable phenomena. In a gap of similarconstruction, however, but in which the central expansion space wasblocked oil by a mica washer, the electrodes did not weld and thedischarge was accompanied by violent arcing, emission of flame from thegap, and excessive heating and burning of the electrodes. Thus, theexpansion space between the electrodes is essential to the satisfactoryoperation of a discharge gap of the type described.

A specific embodiment of the invention has been shown and described forthe purpose of i1- lustration, but it will be apparent that variousmodifications and other constructions are possible within the scope ofthe invention, and it is to be understood, therefore, that the inventionis not limited to the specific embodiment shown, but in its broadestaspects it includes all equivalent modifications and embodiments whichcome within the scope of the appended claims.

W e claim as our invention:

1. A protective discharge gap comprising a pair of flat, relativelythick metal electrodes having opposed plane sparking surfaces, anannular insulating spacer member separating said electrodes with theirsparking surfaces closely spacer so that the electrodes are capable offusing together when a heavy current is discharged, said spacer membercompletely enclosing the space between the electrodes, and at least oneof said electrodes having a central recess of substantial depth thereinto form an expansion space, the volume of said expansion space beinglarge as compared to the volume of the space between the sparkingsurfaces.

2. A protective discharge gap comprising a pair of flat, relativelythick metal electrodes having opposed plane sparking surfaces, anannular insulating spacer member separating said electrodes with theirsparking surfaces closely spaced so that the electrodes are capable offusing together when heavy current is discharged, said spacer membercompletely enclosing the space between the electrodes, and at least oneof said electrodes having a central recess therein, the depth of said rcess being greater than the spacing between the sparking surfaces toform an expansion space of relatively large volume between theelectrodes.

3. A protective discharge gap comprising a pair of fiat, relativelythick metal electrodes having opposed plane sparking surfaces, anannular insulating spacer member separating said electrodes with theirsparking surfaces closely spaced so that the electrodes are capable offusing together when a heavy current is discharged, said spacer membercompletely enclosing the space between the electrodes, one of saidelectrodes having a central recess of greater depth than the spacingbetween the sparking surfaces, the other of said electrodes having acentral opening therethrough corresponding in position to said recess,the recess and opening forming an expansion space between theelectrodes, securing means passing through the recess and opening forholding the electrodes together, and means for closing the opening.

4. A protective discharge gap comprising a pair of relatively thickmetal electrodes having opposed plane sparking surfaces, an annularinsulating spacer member separating said electrodes with their sparkingsurfaces closely spaced so that the electrodes are capable of fusingtogether when a heavy current is discharged, said spacer membercompletely enclosing the space between the electrodes, one of saidelectrodes having a central recess of greater depth than the spacinbetween the sparking surfaces, the other of said electrodes having acentral opening therethrough corresponding in position to said recess,means for closing said opening, an insulating bushing member extendinginto the opening, the bushing being of smaller diameter than theopening, and a bolt passing through the bushing and through a centralhole in the first-mentioned electrode to hold the electrodes together,the bolt being of smaller diameter than said recess and said opening.

EDWARD F. W. BECK.

JOHN S. FERGUSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

Number Number UNITED STATES PATENTS Name Date Barwood 7 Oct. 25, 1938Arco Feb. 20, 1917 Van Depoele Nov. 20, 1883 Young et a1 Apr. 23, 1940Gray Aug. 12, 1930 Janson et a1 1- May 29, 1923 FOREIGN PATENTS CountryDate Great Britain Aug. 2'7, 1925

